1. Field of the Invention
This invention relates to an image forming apparatus such as a copier, a printer, and a facsimile machine.
2. Description of the Related Art
Generally, in a copier using a so-called two-component developing agent consisting of toner and carriers, toner is gradually consumed as developing is proceeded with the result that the toner density, which is a mixing ratio of toner to carrier, lowers. As the toner density is lowered, the density of a copied image is lowered with the result that the quality of image formation may be deteriorated. In view of this, it is necessary to replenish toner by the consumed amount of toner accompanied by development in order to maintain the toner density at a constant level. An example of toner density controlling method is to detect the amount of toner in the developing agent and to replenish toner in such a manner that the detected value is controlled to be a target value.
However, even if the toner density is regulated at a constant value, it is not always the case that the image density for image formation is set at constant level because the number of times of image formation (copying) varies depending on users. It is often the case that a certain correction amount is added to the target value for toner density control.
More specifically, copying demand of users using a copier varies case by case. Some users may demand copying as less as about 1,500 to 3,000 sheets per month. Hereinafter, such a user of less demand is called xe2x80x9clow frequent copy-making user (low-FC user)xe2x80x9d. On the other hand, there may be a user who uses the copier as many as 3,000 sheets or more per month. Hereinafter, such a heavy user is called a high frequent copy-making user (xe2x80x9chigh FC userxe2x80x9d).
It has been customarily believed it desirable for low FC users to use copiers exclusively designed for low FC users (or low FC-oriented copiers) and for high FC users to use copiers exclusively designed for high FC users (or high FC-oriented copiers), wherein the low FC-oriented copier is designed such that a target value of toner density is set at a certain level lower than a level set for the high FC-oriented copier, whereas the high FC-oriented copier is designed such that a target value of the toner density is set at a level higher than the level set for the low FC-oriented copier.
However, such a high FC-oriented copier is expensive, and it is often the case that many users tend to purchase a low FC-oriented copier because they are not certain whether they are a high FC user or a low FC user at the time of purchasing.
Even if a user purchased a low FC-oriented copier, there may be a case that the user is required to make a large number of copying, i.e., using the low FC-oriented copier as the high FC-oriented copier, in the course of using the copier. In such a case, the user encounters a drawback that a sufficient correction amount may not be added to a target value of toner density at the start-up time of the copier with the result that the image density may be lowered as image formation (developing) proceeds, as shown in the dot-dash (chain) line in FIG. 7. It has been believed that occurrence of such a phenomenon is caused by insufficient charging of a photosensitive drum. The insufficient charging of the photosensitive drum can be overcome as time lapses after start-up of the copier, and desirable copying regarding the image density, even when the low FC-oriented copier is used as the high FC-oriented copier, is executable upon lapse of a certain time after start-up of the copier. However, in the conventional low FC oriented copier, the time required for the copier to enable producing the desired image density when used as the high FC-oriented copier, after start-up of the copier cannot be shortened despite the existence of needs by the users.
In view of the above, it is an object of this invention to provide an image forming apparatus that enables to maintain an image density for image formation at a satisfactory level.
In order to achieve the above object, according to an aspect of this invention, an image forming apparatus comprises a toner density detector for detecting a density of toner contained in a two-component developing agent adapted to be used in image formation; a toner density controller for controllably setting (adjusting) the toner density detected by the toner density detector to equal a target value; a mode selector for allowing a user to selectively designate a low frequent copying (FC) mode indicating that the image forming apparatus is in a less-used state where the number of times of image formation is smaller than a predetermined value and a high FC mode indicating that the image forming apparatus is in a highly-used state where the number of times of image formation is not smaller than the predetermined value; and a toner density corrector for correcting the target value of the toner density in such a manner that the toner density controller controllably sets (adjusts) the toner density to equal a target value higher than a target value when the image forming apparatus is in the low FC mode in response to designation of the high FC mode by the mode selector.
In the above arrangement, the toner density corrector corrects the target value of toner density in such a manner that the toner density controller sets the toner density at the target value higher than the target value when the image forming apparatus is in the low FC mode in response to designation of the high FC mode. Accordingly, even if a user uses the image forming apparatus, which is initially set in the low FC mode, at the high FC mode, there is no likelihood that a correction amount to be added to the target value of toner density is insufficient at an initial stage of image formation, thereby lowering image density. In the above arrangement, since image density of a certain desired level is ensured even at an initial stage of image formation, a desirable image quality is ensured in the initial stage of the image formation, and the time required for the apparatus to enable generation of the desired image density even when the apparatus is used as high FC oriented copier after start-up of the apparatus will be shortened and thus the usability of the apparatus is improved. The downside effects due to the insufficient charging of the photosensitive drum in the early stage of the copy making operation are magnified when the apparatus is used in high FC mode, i.e., a large number of copying making operation. However, by setting the apparatus to a high FC mode, i.e., setting a higher toner density target value, at the beginning of the large number copy making operation, the aforementioned downside effects can be alleviated.
These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawing.